Over the past month, scientists at the Louvre have been carefully examining a 16th-century charcoal drawing of a nude woman known as “Monna Vanna,” long attributed to the studio of Leonardo da Vinci. They wondered if, perhaps, there is a closer relationship between this half-smiling nude and the most iconic portrait of all time than experts long supposed.

Since 1862, the “Monna Vanna” drawing has quietly lived in the collection of Renaissance art at the Conde Museum, located in the palace of Chantilly, north of Paris.

The sketch was originally attributed to Leonardo da Vinci when the Duc d’Aumale purchased it for the museum’s collection. However, later analysis suggested the drawing was instead made by one of the artist’s students or assistants in his signature style. Who, exactly, that would have been remained unknown, however.

There are approximately 20 paintings of nude women resembling the “Mona Lisa” around the world, so the likeness alone wasn’t all too unusual. But researchers believed that, given all the “Mona Lisa” nudes out there, da Vinci himself had to have helped with at least one.

“So many students of Leonardo have painted naked Mona Lisas or written about it, that we are almost certain that Leonardo painted one,” Mathieu Deldicque, the deputy curator at the Conde Museum, told The New York Times.

The “Monna Vanna” recently underwent scientific analysis in a laboratory in the basement of the Louvre, in anticipation of a major da Vinci exhibition coming in 2019. The show, marking the 500th anniversary of the artist’s death, is slated to include the nude charcoal sketch.

Analysis by 12 experts found that the fragile rendering was created, at least in part, not only by da Vinci’s assistants, but by the man himself. And what’s more, according to the AFP, experts believe the drawing to be a preparatory study of the iconic “Mona Lisa,” depicting the same sitter sans frock.

If true, the drawing would likely be a rendering of Lisa Gherardini, long thought to be the “Lisa” of “Mona Lisa” glory. The famed oil portrait was commissioned by Florentine silk merchant Francesco del Giocondo, Gherardini’s husband.

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For the past 500 years, the “Mona Lisa” has enchanted art lovers with its lush, earthy palette and the sitter’s enigmatic gaze. The charcoal rendering seems to be just as captivating.

“The drawing has a quality in the way the face and hands are rendered that is truly remarkable,” Deldicque told the AFP. “It is not a pale copy. We are looking at something that was worked on in parallel with the ‘Mona Lisa’ at the end of Leonardo’s life.”

He added, “It is almost certainly a preparatory work for an oil painting.”

The evidence is as follows: First, the drawing and the painting are the same size, 77 by 53 centimeters. Second, the hands and body in both the “Monna Vanna” and “Mona Lisa” are rendered in exactly the same position and from the same angle.

Finally, small holes pierced around the charcoal figure suggest Leonardo may have traced the outline of the drawing onto a canvas. This outline, then, could have served as the starting point of a painting.

The timing supporting this theory adds up. While the “Monna Vanna” is thought to have been created between 1514 and 1516, the “Mona Lisa” is dated between 1503 and 1519.

The most damning detail against the scientists’ theory, however, is the hatching on the top of the drawing, which appears to be created by a right-handed person, according to The New York Times. Da Vinci was left-handed.

The hatches on the bottom of the drawing, however, are harder to make out. Therefore, it’s possible da Vinci worked on the lower half of the drawing while one of his right-handed students worked up top.

“We are sure of nothing, and if Leonardo participated, it’s not for all the drawing, but for some parts of it,” Deldicque told The Times.

Louvre conservation expert Bruno Mottin explained the necessity of not rushing to conclusions. “We must remain prudent,” he told the AFP. “It is a very difficult drawing to work on because it is particularly fragile.”

Another day, another “Mona Lisa” mystery. The woman sure knows how to maintain her mystique.